Chromium Plating

ABSTRACT

A method of chromium coating a substrate is described, which method comprises: a) depositing, preferably electrodepositing, a metallic layer onto the substrate comprising chromium from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium; b) physical vapor depositing a layer comprising chromium cn the layer deposited from step a). In one embodiment, in step a), the layer is chromium and is electrodeposited from a plating bath that includes trivalent chromium and no, or substantially no, hexavalent chromium. The metallic layer deposited in step a) is preferably a metal or an alloy of two or more metals and may be selected from the group consisting of chromium, copper, nickel, cobalt, zinc, lead iron, palladium and tin and alloys thereof, e.g. copper tin, copper zinc, tin lead, tin nickel, tin cobalt, tin zinc, iron nickel, cobalt nickel, zinc nickel, zinc iron, zinc iron nickel, zinc iron cobalt or palladium nickel. Copper and nickel are especially preferred. The coating formed by the method has a blue sheen that is usually associated with chromium electrodeposited from hexavalent chromium baths without having to use a hexavalent chromium bath, which is highly toxic.

TECHNICAL FIELD

The present invention relates to chromium plating of substrates,including metal and plastic substrates.

BACKGROUND ART

Chromium plating is a well known technique for protecting substrates andgiving them an attractive appearance. It is known to electroplate bothmetallic and plastic substrates using an electroplating bath containinghexavalent chromium, typically chromic acid or chromates (H₂CrO₄ andCrO₄ ²⁻). Electroplated deposits from hexavalent plating baths provide astrong, abrasion-resistant layer that will protect the underlyingsubstrate from corrosion and provide an attractive decorative finish.One feature that is seen to be particularly attractive is the “bluesheen” of the chromium layer that is electrodeposited from hexavalentchromium baths.

Hexavalent chromium, however, is believed to be toxic and carcinogenicand its use is being phased out. Accordingly, alternatives are beingsought. One alternative that has been proposed is a bath based ontrivalent chromium, which does not give rise to the same health problemsas hexavalent chromium, see for example U.S. Pat. No. 5,415,763, U.S.Pat. No. 5,413,646 U.S. Pat. No. 524,326 and U.S. Pat. 5,196,109.

However, trivalent chromium electroplating baths are not stable and aredifficult to control. Thus it is not easy to obtain a consistentlysmooth finish using a trivalent chromium bath; furthermoreelectrodeposits from trivalent chromium baths do not have thecharacteristic blue sheen of electrodeposits from hexavalent chromiumbaths, which is seen to be highly desirable.

Physical vapor deposition (often abbreviated “PVD”) is a well knowntechnique for forming layers on substrates and involve the evaporationof the coating material by bombarding a target with a high energysource, such as a beam of electrons or ions. This beam dislodges atomsfrom the surface of the target thereby vaporising them. The vaporisedatoms are transported from the target to the substrate, where they forma deposit on the substrate.

JP-63-230189 describes a cutter for a razor in which the outer part ofthe cutter blade has a nickel layer formed by electrodeposition from anickel sulphamate bath, a layer of a phosphorus-containing alloy, e.g.Ni—P, and a layer of chromium less than 0.2 μm thick and formed bysputtering.

U.S. Pat. No. 4,906,533 describes a method of plating steel for use inmaking tin cans; the steel is electroplated with chromium, removingchromium oxide from the deposited layer and depositing a layer ofaluminium by vacuum deposition.

CH 660278 describes a steel watch case that is electroplated withchromium and the a layer of TiC is applied by PVD.

JP 6-192853 and JP 3-197688 describe a method of applying a hardenedsurface to an object by electroplating it with chromium and applying aceramic layer, e.g. TiN, by PVD.

JP 59-215483 describes a method of applying a hardened gold-colouredsurface to an object by electroplating it with chromium and sputteringTi onto in a nitrogen atmosphere to form a gold-coloured TiN layer.

DE-A-1514668 describes a method of making electrical contacts forsemrconductors by depositing successively by PVD a Cr layer, a Cr and Aglayer and a layer of Ag alone; the layers can be thickened byelectrodeposition.

GB 2327091 describes a method of applying a decorative and protectivecoating on an object by electroplating a layer of Cu, Ni or Cr, e.g.from a hexavalent chromium bath, pulse blow drying the electrodepositedlayer and depositing a refractory metal (Zr, Ti, Hf or Zr/Ti) by PVD.The method can produce a brass colour.

DISCLOSURE OF INVENTION

It has now been found that coatings having a blue sheen can be obtainedwithout using hexavalent chromium baths.

According to the present invention, there is provided a method ofcoating a substrate to provide a decorative and protective chromiumdeposit having a blue sheen without having to use hexavalent chromiumelectrodeposition, which method comprises:

-   -   a) depositing a metallic layer onto the substrate, and;    -   b) physical vapor depositing a layer comprising chromium on the        layer deposited from step a).

Preferably in step a), the layer is chromium and is electrodepositedfrom a plating bath that includes trivalent chromium and no, orsubstantially no, hexavalent chromium.

The metallic layer deposited in step a) is preferably a metal or analloy of two or more metals and may be selected from the groupconsisting of chromium, copper, nickel, cobalt, zinc, lead iron,palladium and tin and alloys thereof, e.g. copper tin, copper zinc, tinlead, tin nickel, tin cobalt, tin zinc, iron nickel, cobalt nickel, zincnickel, zinc iron, zinc iron nickel, zinc iron cobalt or palladiumnickel. Copper and nickel are especially preferred. Preferably the layerdeposited in step a) contains at most only traces of non-metals that arecommon in the electroplating art, e.g. nickel deposits often containsulphur.

The invention is primarily directed at decorative and protectivecoatings rather than coatings to provide hardness and wear resistancethat are used in cutting and tooling applications and so the layersdeposited on the substrate, including in step a), preferably do notinclude a hard ceramic, e.g. a nitride or phosphide, or a phosphoruscontaining alloy.

It has also been found that the method of the present invention providesa chromium coating that is highly effective in preventing corrosion ofthe substrate.

Preferably, the electrodeposited chromium layer (step a) has a thicknessof 0.1 to 1 μm, e.g. 0.3 to 0.9 μm, such as 0.5 to 0.8 μm. Preferablythe step a) layer is greater than 0.2 μm thick. The preferred thicknessof the PVD layer is 0.01 to 2 μm, e.g. 0.05 to 1.5 μm, such as 0.1 to 1∞m.

Before the article is coated in step a), e.g. electroplated withchromium, it may be provided with one or more protective or barrierlayers, e.g. a base layer of nickel, preferably formed byelectrodeposition. If the substrate is plastic, its surface should berendered conductive, which can be achieved using well-known techniquesfor electroplating plastic and other non-conductive substrates.

It is not necessary that the chromium PVD deposit is formed directly onthe metal, e.g. chromium, deposited in step a) and an intermediatelayer, e.g. of white metal such as nickel, may be laid down between thestep a) layer and the PVD layer.

Both electrodeposition from trivalent chromium baths and PVD arewell-known techniques and so their operation will be entirely apparentto the skilled person.

The substrate may be plastic. Electroplating of plastic substrates iswell-known and involves subjecting the insulating plastic to a series ofknown pre-treatment steps that render the surface electricallyconductive. These steps involve the non-electric deposition of aconductive material, e.g. nickel, and the deposition of a layer ofnichrome (nickel-chromium alloy). The metallic layer may be thickenedthrough electroplating of cupronickel, using the following processsteps:

-   -   pre-treatment→acid cleaning water→cleaning→pyrophosphoric        copper→water cleaning→acid cleaning→bright copper→water        cleaning→activation→water cleaning→semi-bright nickel→bright        nickel→seal nickel→water cleaning→drying

The plastic has thereby become conductive and is suitable for generalelectroplating.

BEST MODE FOR CARRYING OUT THE INVENTION

Although electrodeposition from a trivalent chromium bath and PVD arewell-known individually, we have found that the following are theoptimum conditions for achieving the required blue sheen to a chromiumdeposit using the present invention. However, these conditions are notintended to limit the claims.

A substrate may be pre-treated with one or more base layers, e.g. atleast one nickel layer; in the case of a substrate made of plastic, anelectrically conductive surface layer must be applied to the insulatingplastic surface as described above. The substrate may be, for example, aplastic moulding, a zinc die cast, a cast or machined brass part or amachined steel part. The substrate is then subjected to trivalentchromium plating followed by PVD of chromium under the conditionsdescribed below:

Trivalent Chromium Plating Bath composition: trivalent chromiumTemperature:  20° C.-50° C. Bath Acidity (pH): 2.8-3.8 Current Density:  5-20 A/dm² Electroplating Time:   2-6 mins Thickness of theelectrodeposit: 0.1-1 μm

The resulting chromium electrodeposit 99.99% Metallic Chromium meets thefollowing tests as a result of the trivalent chromium plating process:—

(1) Temperature test: 72° C.→ 25° C.→ −30° C. (3 cycles) (2) CASS test:8-16 HR

Physical Vapor Deposition (PVD)

Technique: Electric Arc Style Physical Vapor Deposition Material beingdeposited: Metallic Chromium Deposit Content: 99% Metallic ChromiumDeposition Temperature: <70° C. (plastic substrate)   70-120° C. (Zincdie cast substrate)  120-180° C. (Brass, Steel or other materialssubstrate) Deposit thickness: 0.01-2 μm, preferably 0.1-0.5/μm Pressure:2 × 10⁻⁵− 2 × 10⁻⁷ Torr Colour: Silver-white Process Steps: Venting (2 ×10 Torr)   20-30 mins glow discharge   5-10 mins high bias   1-3 minsmetal layer (99.99% Chromium)   1-100 mins

The resulting deposit had a blue sheen. The “L*” (black to white), “a*”(green to red), “b*” (blue to yellow) values of the colour measurementfor the blue sheen effect of chromium are:

i. Measurement Instrument: Spectrophotometer ii. Setting of theInstrument: D65/10, SCI iii. L*   82.4 to 88.4 iv. a*  −1.5 to 0 v. b*  −2 to 0

1. A method of chromium coating a substrate to provide a chromiumdeposit having a blue sheen, which method comprises: a) depositing ametallic layer on the substrate; and b) physical vapor depositing alayer comprising chromium on the layer deposited from step a).
 2. Amethod as claimed in claim 1 wherein step a) comprises electrodepositinga layer comprising chromium from a plating bath that includes trivalentchromium and no, or substantially no, hexavalent chromium.
 3. A methodas claimed in claim 1 wherein the metal deposited in step a) is a singlemetal or an alloy of two or more metals.
 4. A method as claimed in claim3, wherein the metal deposited in step a) is selected from the groupconsisting of chromium, copper, nickel, cobalt, zinc, lead iron,palladium and tin and copper tin, copper zinc, tin lead, tin nickel, tincobalt, tin zinc, iron nickel, cobalt nickel, zinc nickel, zinc iron,zinc iron nickel, zinc iron cobalt or palladium nickel.
 5. A method asclaimed in claim 3, wherein the metal deposited in step a) is selectedfrom chromium, copper and nickel.
 6. A method as claimed in claim 1,wherein the metal deposited in step a) is electrodeposited.
 7. A methodas claimed in claim 1, wherein the metal deposited in step a) has athickness of 0.05 to 1 μm.
 8. A method as claimed in claim 7, whereinthe metal deposited in step a) has a thickness of 0.3 to 0.9 μm.
 9. Amethod as claimed in claim 8, wherein the metal deposited in step a) hasa thickness of 0.5 to 0.8 μm.
 10. A method as claimed in claim 1,wherein the vapor deposited chromium layer (step b) has a thickness of0.05 to 1.5 μm.
 11. A method as claimed in claim 11, wherein the vapordeposited chromium layer (step b)) has a thickness of 0.05 to 1 μm. 12.A method as claimed in claim 12, wherein the layers vapor depositedchromium layer (step b)) has a thickness of 0.1 to 1 μm.
 13. A method asclaimed in claim 1, wherein the layers formed in steps a) and b) consistsubstantially wholly of chromium and the layer formed in step a) isformed by electrodeposition.
 14. A method as claimed in claim 1, whereinstep b) deposits the chromium layer directly on top of the layer formedin step a).
 15. A product having a chromium coating made by the methodof claim 1.